Water Transport Coefficient Research Articles

Self-Assembly, Transport, and Thermodynamics in Nafion Membranes: Insight from Dissipative Particle Dynamics Simulation

Composed of perfluorinated backbone with sulfonate sidechains, Nafion self-assembles upon hydration and segregates into interpenetrating hydrophilic and hydrophobic subphases. This segregated morphology determines the transport properties of Nafion membranes that are widely used as compartment separators in fuel cells and other electro-chemical devices, as well as permselective diffusion barriers in protective fabrics. We introduce a coarse-grained model of Nafion, which accounts explicitly for polymer rigidity and electrostatic interactions between anionic sidechains and hydrated metal cations. By means of dissipative particle dynamics (DPD) and Monte Carlo (MC) simulations, we explore geometrical, transport, and sorption properties of hydrated Nafion-type polyelectrolyte membranes. Novel methodology will be presented for calculating water adsorption isotherms and transport coefficients as a function of the degree of hydration.

A CNT (carbon nanotube) paper as cathode gas diffusion electrode for water management of passive μ-DMFC (micro-direct methanol fuel cell) with highly concentrated methanol

A novel MEA (membrane electrode assembly) structure of passive μ-DMFC (micro-direct methanol fuel cell) controls water management and decreases methanol crossover. The CNT (carbon nanotube) paper replacing CP (carbon paper) as GDL (gas diffusion paper) enhances water back diffusion which passively prevents flooding in the cathode and promotes low methanol crossover. Moreover, the unique structure of CNT paper can also enhance efficiency of oxygen mass transport and catalyst utilization. The passive μ-DMFC with CNT-MEA exhibits significantly higher performance than passive μ-DMFC with CP-MEA and can operate in high methanol concentration, showing the peak power density of 23.2 mW cm−2. The energy efficiency and fuel utilization efficiency are obviously improved from 11.54% to 22.7% and 36.61%–49.34%, respectively, and the water transport coefficient is 0.47 which is lower than previously reported passive μ-DMFC with CP.

Characterization of Two Different Stumps of <i>Spirulina platensis</i> Drying: Assessment of Water Transport Coefficient

The sorption behaviour and water transport mechanisms inside Spirulina platensis samples were experimentally analysed during isothermal drying at 25℃ and 50℃. Two different products grown in semi-industrial farms from Burkina Faso and France with initial water contents respectively of the range from 2.73 kg w /kg dm to 3.12 kg w /kg dm were characterized. A novel procedure has been developed to determine the water content profiles inside samples during isothermal drying. At both temperatures, experimental results underlined that the physical properties of Spirulina are not sensitive to the geographical origin, Burkina-Faso or France. To keep Spirulina at an water activity below 0.6 in order to preserve it from micro-organisms development, sorption isotherm curves show that a sufficient requirement is to lower the water content until an upper limit of w = 0.075 db. The evolution of water transport coefficient as a function of water content highlights a monotonous exponential dependence with a transport coefficient ranging from 1.70 × 10–10 to 94 × 10–10 m2/s. The contribution of solid phase shrinkage to the transport of water is negligible for the last drying steps.

Two-dimensional modeling of a polymer electrolyte membrane fuel cell with long flow channel. Part I. Model development

A two-dimensional single-phase model is developed for the steady-state and transient analysis of polymer electrolyte membrane fuel cells (PEMFC). Based on diluted and concentrated solution theories, viscous flow is introduced into a phenomenological multi-component modeling framework in the membrane. Characteristic variables related to the water uptake are discussed. A Butler–Volmer formulation of the current-overpotential relationship is developed based on an elementary mechanism of electrochemical oxygen reduction. Validated by using published V–I experiments, the model is then used to analyze the effects of operating conditions on current output and water management, especially net water transport coefficient along the channel. For a power PEMFC, the long-channel configuration is helpful for internal humidification and anode water removal, operating in counterflow mode with proper gas flow rate and humidity. In time domain, a typical transient process with closed anode is also investigated.

Experimental and modeling studies of a micro direct methanol fuel cell

The Direct Methanol Fuel Cell (DMFC) has attracted much attention due to its potential applications as a power source for transportation and portable electronic devices. Based on the advantages of the scaling laws, miniaturization promises higher efficiency and performance of power generating devices and the MicroDMFC is therefore an emergent technology. In this work, a set of experiments with a MicroDMFC of 2.25 cm2 active area are performed in order to investigate the effect of important operating parameters. Maximum power density achieved was 32 mW/cm2 using a 4 M methanol concentration at room temperature. Polarization curves are compared with mathematical model simulations in order to achieve a better understanding of how parameters affect performance. The one-dimensional model used in this work takes in account coupled heat and mass transfer, along with the electrochemical reactions occurring in a direct methanol fuel cell and was already developed and validated for DMFC in previous work by Oliveira et al. [1–3]. The model is also used to predict some important parameters to analyze fuel cell performance, such as water transport coefficient and methanol crossover. This easy to implement simplified model is suitable for use in real-time MicroDMFC simulations. More experimental data are also reported bearing in mind the insufficient experimental data available in literature at room temperature, a goal condition to use this technology in portable applications.

Real Time Measurement of Water Transport in Polymer Electrolyte Fuel Cell

Water management is a crucial aspect in operating a polymer electrolyte fuel cell. The dynamic behaviors of the water transport coefficient and electric potential were obtained from experiments. The real-time water transport coefficient α was observed by measuring the relative humidity of the inlet and outlet gases. The α passed through three steps from start-up to shut-down. The steady state α increased with increasing oxygen gas flow rate due to the water generated in the cathode catalyst layer more significantly exhausted into the gas channel and the effect of back diffusion was limited. The degradation of the performance at the higher transport coefficient might be due to the dehydration of the anode side of the membrane. During the start-up process, a potential undershoot was observed. Based on the result of the real-time α measurement, the anode side gradually was hydrated and the membrane resistance decreased because the water moved from the cathode to the anode. In the load change experiment, the peak value of αa after the load change and the potential undershoot decreased with increasing current of the first step. It was expected that the anode side was more dehydrated by the load change at a lower current in the first step. Finally, the experimental α at a steady state was in good agreement with the simulated results.

Separation Analysis of Electro-Osmosis and Diffusion of Water in Electrolyte Membrane of PEMFC under Low-Humidity Operation

To alleviate performance degradation of proton exchange membrane fuel cell (PEMFC) due to membrane dehydration during low-humidity operation, it is essential to understand and control water transports through electrolyte membrane. In this study, the water vapor and current distributions on the anode side of a PEMFC was firstly measured by using the effective visualization tool proposed in our previous work, and the net water transport coefficient was quantitatively estimated under low-humidity conditions. Furthermore, the electro-osmotic drag (EOD) coefficient of Nafion membrane was also measured based on hydrogen pumping method. Therefore, the electro-osmosis and back-diffusion of water through the electrolyte membrane in the operating PEMFC can be separately analyzed. Results revealed that the cathode inlet humidification promotes the back-diffusion of water rather than the electro-osmotic flow, and prevents the membrane dryout.

Water transport in parchment and endosperm of coffee bean

This paper aims at contributing to identify the eventual regions where fungus Aspergillus ochraceus could grow and produce ochratoxin A (OTA) during drying of coffee beans. Internal structure of coffee bean was analyzed by optical microscopy for endosperm and parchment. From the expression of the dissipation in the grain due to the water transport, we show that a relationship formally analogous to an equation of diffusion governs the water transport. Three structures with mass transfer resistance potential are studied: parchment, silver skin and endosperm. An experimental technique to study the water transfer into the parchment was proposed. In the endosperm, for moisture contents above 65%, a constant transport coefficient controls the drying kinetics of the whole bean. Below this moisture content, water transport coefficient (with and without silver skin) were significantly lesser than those for the whole bean. This is firstly due to the reduction of the pore space occupied by water and second to the increasing bonding energy between solid structure and water as moisture content decreases. The contribution of parchment to the protection of the endosperm is highlighted.

One‐dimensional Model of Oxygen Transport Impedance Accounting for Convection Perpendicular to the Electrode

AbstractA one‐dimensional (1D) model of oxygen transport in the diffusion media of proton exchange membrane fuel cells (PEMFC) is presented, which considers convection perpendicular to the electrode in addition to diffusion. The resulting analytical expression of the convecto‐diffusive impedance is obtained using a convection–diffusion equation instead of a diffusion equation in the case of classical Warburg impedance. The main hypothesis of the model is that the convective flux is generated by the evacuation of water produced at the cathode which flows through the porous media in vapor phase. This allows the expression of the convective flux velocity as a function of the current density and of the water transport coefficient α (the fraction of water being evacuated at the cathode outlet). The resulting 1D oxygen transport impedance neglects processes occurring in the direction parallel to the electrode that could have a significant impact on the cell impedance, like gas consumption or concentration oscillations induced by the measuring signal. However, it enables us to estimate the impact of convection perpendicular to the electrode on PEMFC impedance spectra and to determine in which conditions the approximation of a purely diffusive oxygen transport is valid. Experimental observations confirm the numerical results.

F114 低加湿運転PEFCのストイキ比及び加湿条件が膜厚方向の水分輸送に及ぼす影響(OS-12: 燃料電池関連研究の新展開(1))

To alleviate membrane dryout in PEFCs under low-humidity operation, it is essential to understand and control water crossover through electrolyte membrane. This paper presented a novel method for quantitatively measuring the water vapor distribution in the gas channel of a low-humidity PEFC using humidity test paper (HTP), and investigated the effects of stoichiometry and gas humidification on the water transport in the anode. Furthermore, the flow distributions of hydrogen and water vapor on the anode side were estimated based on current distribution measurement, and the local water transport coefficient through the membrane was numerically predicted. It was found that the low stoichimetric cathode flow enhances the back-diffusion of product water across the membrane, and suppresses the membrane dehydration.

Verification of The Water Transport Parameter - Moisture Storage Function of Autoclaved Aerated Concrete - Approximately Calculated from a Small Set of Measured Characteristic Values

The aim of the paper is to determine the moisture transport parameters for selected type of autoclaved aerated concrete using fly ash order to carry out the numerical analysis of the hygrothermal performance of selected building structures in real conditions. The moisture storage function (sorption, suction) and the liquid water transport coefficient are taken into consideration. Obtaining knowledge of these parameters involves a series of laboratory measurements and tests. Applying those parameters into modern calculation methods (WUFI) we will be able to analyse annual hygrothermal performance of porous concrete wall constructions in climatic conditions of Central Europe. This article aims to test use of a simplified approximating method to establish the moisture storage function for a chosen building material. Influences of the method on accuracy of the simulated calculation will also be analysed.

Experimental Study of Effects of Operating Conditions on Water Transport Phenomena in the Cathode of Polymer Electrolyte Membrane Fuel Cell

Water management in polymer electrolyte membrane fuel cells is important because fuel cell performance may be lower when flooding emerges. In addition, the proton conductivity and water transport coefficient in the membrane depend on the hydration of the membrane. In this study a water transport phenomenon in the cathode channels of a polymer electrolyte membrane fuel cell was investigated under various operating conditions. To obtain images of the water the transparent fuel cell had a polycarbonate window installed at the cathode end plate, and gold-coated stainless steel was used for the flow field and current collector of the cathode. The effects of operating conditions on water transport manipulated operating parameters such as cell temperature, cathode flow rate, and cathode backpressure. As the operating time elapsed, it was observed that water droplet formation, growth, coalescence, and removal occurred in the cathode channel. It concluded that a high cathode flow rate prevented flooding by removing water from the cathode flow channel. Also, the quantity of water droplets increased with a high cathode backpressure.

The real time dynamic water transport investigation due to net water transport coefficient (NWTC) based on measurement of water content in the outlet gas in a PEFC

The present method is expanded from the previous research [1], based on the experimental water pressure in the anode and cathode outlet. The real time NWTC in cathode and anode are obtained. The general NWTC, the peak NWTC, and the average NWTC in start-up and shut-down are discussed. The results show that, firstly, the membrane absorbs water from cathode and anode in the start-up after the current turned on and discharges water to cathode and anode after the current turned off, which causes the dynamic NWTC far from the steady state value. Secondly, there is peak value for NWTC both for start-up and shut-down, the anode peak value is nearly independent on the current in start-up, and decreases (more water transport) with increase of the current in shut-down. The asynchronous appearance moment of the peak values in the cathode side and the anode side are obtained. The average NWTC in the time interval of start-up keeps the same trend as the steady NWTC with increase of the current in start-up, and more water is discharged from the membrane for high current in shut-down.

A pseudo-phase-equilibrium approach for the calculation of liquid water saturation in the cathode gas diffuser of proton-exchange-membrane fuel cells

The cathode compartment of membrane electrode assembly (MEA) of the proton-exchange-membrane fuel cell (PEMFC) is studied theoretically. A pseudo-phase-equilibrium approach adopting an approximate phase-equilibrium equation, incorporated with the associated equations of gaseous multi-component diffusion, liquid water capillary transport, and surface electrochemical kinetics, well characterizes the performance of cathode electrode under unsaturated feed. The pseudo-phase-equilibrium approach avoids the need of explicit liquid water front tracking so that only a single domain formulation, without consideration of the interior boundary, is required for the simulation. In order to illustrate the capability of the proposed approach, a mathematical model of the cathode compartment of MEA in one-dimension is formulated, in which gas species concentration profiles, liquid water distribution, and liquid water front are calculated. The validity of the pseudo-phase-equilibrium approach is then evaluated over an extensive polarization range under specified operating temperature, pressure, and inlet humidity. The solutions obtained using the pseudo-phase-equilibrium approach and the exact phase-equilibrium equation are compared over a wide range of parameter values. In addition, the influences of important transport parameters such as water transport coefficient, gas diffuser porosity, and absolute liquid permeability are evaluated and discussed.

Effects of Water Vapor Proportion in Generated Water on Performance of Polymer Electrolyte Fuel Cells under Dry Operating Conditions

A one-dimensional, non-isothermal, single-phase, steady-state model is developed for dry operating conditions to investigate the effects of different types of generated water as liquid or vapor in electrochemical reaction on the proton exchange membrane fuel cell (PEMFC) performance, A parameter (Γ) denoting vapor phase fraction in generated water is employed in the model. The agglomerate model with thin film was employed in catalyst layers. Water transport and its effects on cell performance were discussed under different operating conditions such as reactants relative humidity, vapor phase fraction and micro-porous layer thickness. The results indicated that the net water transport coefficients were positive under equal feed gas humidity in both anode and cathode. At high current density, the more the proportion of water vapor in generated water is, the better cell performance is. However, at low current density, the effect of vapor phase proportion on cell performance is not very obvious.

Water Balance Simulations of a PEM Fuel Cell Using a Two-Fluid Model

A previously published computational multi-phase model of a polymer-electrolyte membrane fuel cell has been extended in order to account for the anode side and the electrolyte membrane. The model has been applied to study the water balance of a fuel cell during operation under various humidification conditions. It was found that the specific surface area of the electrolyte in the catalyst layers close to the membrane is of critical importance for the overall water balance. Applying a high specific electrolyte surface area close to the membrane (a water-uptake layer) always leads to a lower net water transport coefficient. Thus we can reduce flooding at the cathode and may obtain improved cell performance due to a better humidified membrane. The results also suggest that membrane dehydration may occur at either anode or cathode depending on the net water transport.

Fluorination of Vulcan XC-72R for cathodic microporous layer of passive micro direct methanol fuel cell

The effect of adding fluorinated Vulcan XC-72R into the microporous layer (MPL) of the cathode in a passive micro direct methanol fuel cell (μDMFC) has been investigated. Upon fluorination with fluoro-alkyl silane (FAS), the surface of XC-72R becomes more hydrophobic, as indicated by contact angle measurements. The performance of the membrane electrode assembly (MEA) is improved significantly when fluorinated Vulcan XC-72R is used in MPL of the cathode. The maximum power density of a passive μDMFC reached ca. 36.2 mW cm−2 at room temperature, and the constant-current discharging test exhibits enhanced stability. Also observed is a decreased water transport coefficient (α), calculated from discharging test, attributable to the greater hydrophobicity resulting in higher liquid pressure on the cathode, which forces more water to flow back to the anode. Additionally, A.C. impedance analysis indicates that the improvement in performance results from the decrease of charge transfer resistance of the cathodic reaction.

Numerical investigation of the impact of gas and cooling flow configurations on current and water distributions in a polymer membrane fuel cell through a pseudo-two-dimensional diphasic model

For optimal performances, proton exchange membrane fuel cells require fine water and thermal management. Accurate modelling of the physical phenomena occurring in the fuel cell is a key issue to improve fuel cell technology. Here, an analytic steady state diphasic 2D model of heat and mass transfer is presented. Through this model, the aim of this work is to study the influence of local events on the global performances of a fuel cell. A part of the complete model is a microscopic representation of the coupling between water transport and charge transfers in the electrodes. The thickness of the liquid layer around the reactive agglomerates is deduced from the saturation. The evolution of the quantity of water within the catalyst layer is monitored and its influence on the global performances of the cell is investigated. In gas diffusion layers (GDLs), liquid and vapour water transport through are computed regarding the temperature. The flow direction of cooling water modifies the current density distribution along the cell. The impact of the direction of air and hydrogen feeding channels are investigated. It can modify greatly the fuel cell mean current density and the net water transport coefficient. The counter-flow mode was preferable. Likewise, thanks to a better membrane hydration, it results in independent performances regarding the hydrogen inlet relative humidity or stoichiometry.

The real-time determination of net water transport coefficient based on measurement of water content in the outlet gas in a polymer electrolyte fuel cell

A numerical approach is developed to determine the real-time Net Water Transport Coefficient (NWTC) based on the experimental water vapor pressure for the cathode and anode outlet obtained by the optical humidity sensors with Tunable Diode Laser Absorption Spectroscopy (TDLAS). The results show that there are sharp vibrations for NWTC in the process of start-up and shut-down. And the time needed for the water transport balance increases with the increase in the current. The balanced NWTC ranges from −0.2 to 0.2, and it increases with the increase in the operation current in the present research. In the view of flooding prevention, it is reasonable to humidify the anode inlet gas with the lower temperature than that of cathode side by decreasing the osmotic-drag water from anode to cathode.

Numerical Analysis of Water Transport Through the Membrane Electrolyte Assembly of a Polymer Exchange Membrane Fuel Cell

The effects of water transport through membrane electrolyte assembly of a polymer exchange membrane fuel cell on cell performance has been studied by a one-dimensional, nonisothermal, steady-state model. Three forms of water are considered in the model: dissolved water in the electrolyte or membrane, and liquid water and water vapor in the void space. Phase changes among these three forms of water are included based on the corresponding local equilibriums between the two involved forms. Water transport and its effect on cell performance have been discussed under different operating conditions by using the value and the sign of the net water transport coefficient, which is defined by the net flux of water transported from the anode side to the cathode side per proton flux. Optimal cell performance can be obtained by adjusting the liquid water saturation at the interface of the cathode gas diffusion layer and flow channels.